This invention relates to Dense Wavelength Division Multiplexer (DWDM) devices, and more particularly to DWDM cascade tree structures that provide specified flat-top passband response and optical isolation.
A DWDM device can be used to increase the number of communication channels available in a fiber optical system. Today, researchers are studying a few competing technologies. One mature technology, relying on standard thin film filter coatings, is characterized by high signal insertion losses, low channel counts and relatively high cost, and is generally useful only for DWDM devices with channel spacing greater than 50 GHz.
Another competing technology, involving use of Mach-Zehnder interferometers (xe2x80x9cMZIsxe2x80x9d), is characterized by low signal insertion loss, low polarization dependent low, relatively low cost, high uniformity and high signal crosstalk and is a more attractive choice for DWDM devices with lower channel spacing. However, standard MZI technology suffers from low isolation between adjacent channels and provides an approximately Gaussian shape for the corresponding transmission curves. These latter two problems make it difficult for a DWDM device relying on standard MZI technology to comply with DWDM standards for optical isolation and xe2x80x9cflat-topxe2x80x9d passband response set down by BellCore. If these two problems can be either solved or reduced in severity, DWDM devices relying on MZI technology could become widely used in voice, data and image communications.
What is needed is a DWDM system having low signal insertion loss, low polarization dependent loss, high uniformity, relatively low signal crosstalk, acceptable channel isolation and acceptably low passband insertion loss. Preferably, the system should have acceptably low cost and should be flexible enough to meet various commercial communication requirements.
Preferably, the system should meet or exceed the BellCore standards for flat-top passband response and for optical isolation.
These needs are met by the invention, which uses an improved DWDM cascade structure with distributed filtering and MZI technology to provide acceptable channel isolation for relatively low channel spacing and to comply with the BellCore standards for optical isolation and for flat-top passband response within a channel, over a system of 2N output channels for 100 GHz (or higher) channel spacing with N=4, 5, 6, . . .
The basic structure is a bifurcated or cascade tree system with N stages, numbered n=0, 1, 2 . . . , N, with stage number 0 being a light input channel, with stage number n having 2n fiber optical channels in parallel, with each channel in stage n (1xe2x89xa6nxe2x89xa6N) having an MZI, defined by two 3 dB couplers and two parallel fiber optic arms of unequal length, at the beginning of the channel, and with each channel except an output channel or port feeding an MZI that is part of stage n+1 (n=1, 2 . . . , Nxe2x88x921). A xe2x80x9cstagexe2x80x9d, as used herein, refers to a group of one or more parallel fiber optic channels, with each channel having an MZI positioned at the beginning of the channel for wavelength discrimination. A typical cascade tree structure of fiber optic channels is disclosed and discussed in U.S. Pat. Nos. 5,809,190 and 5,987,201, issued to P. Z. Chen (FIG. 1 and discussion), incorporated by reference herein. Use of a Fabry-Perot etalon as a filter in a fiber optic channel is disclosed by Colbourne in U.S. Pat. No. 5,666,225, incorporated by reference herein.
Optical isolation complying with the present BellCore standards for specified channel spacings can be approached by careful design and fabrication of the MZIs used in the cascade structure. However, a straightforward approach sharpens the peaks of the transmission factors, as a function of wavelength, so that compliance with the required flat-top passband responses becomes even more difficult.
The invention provides combines the basic structure with either a filter that is either an inverted non-symmetric MZI (xe2x80x9cNSMZIxe2x80x9d) or with an inverted Fabry-Perot etalon (xe2x80x9cFPExe2x80x9d) having a modest optical finesse (Fxe2x89xa62) that is incorporated in all channels in a selected stage, preferably stage 0, 1, 2 or 3, where a sequence of transmission minima for the inverted filter coincides with a selected sequence of wavelengths that substantially coincide with the peak wavelengths, in order to facilitate a xe2x80x9cflat-topxe2x80x9d response that complies with the BellCore standards.